In a magnetic recording medium, higher recording density is increasing by desired. A known technique for obtaining a higher recording density is to provide a smooth magnetic layer of a coating type magnetic recording medium. A coating type magnetic recording medium is prepared by coating a non-magnetic support with a magnetic coating composition composed mainly of a ferromagnetic powder and a binder.
In a second technique for obtaining a higher recording density, a ferromagnetic thin film type magnetic recording medium has been developed having a surface of the magnetic layer that is smoother than that of a coating type magnetic recording medium. A ferromagnetic thin film type magnetic recording medium is hereinafter referred to as a "metal thin film type magnetic recording medium".
In the above described recording media, however, if the surface of the magnetic layer is smoothened to improve the electromagnetic characteristics, the coefficient of friction between the magnetic layer and an apparatus (e.g., a video head), is increased during running of the magnetic recording medium. As a result, the magnetic layer of the magnetic recording medium is deteriorated over a short period of use. In an extreme case, the magnetic layer may peel from the support.
In order to solve the above problem, it is known to decrease the coefficient of friction of the magnetic layer surface, for example, by adding a lubricant to the magnetic coating composition or by coating a lubricant onto the surface of the magnetic layer.
As such lubricants, mineral oil, silicone oil, higher alcohols, higher fatty acids, fatty acid esters, animal oils such as cow wax, whale oil and shark oil, and plant oils have heretofore been used.
When the smoothness of the surface of the magnetic layer is further increased, a higher lubricating effect is required. As the addition amount of the above described lubricant is increased, the strength of the magnetic layer is decreased. A decrease in the mechanical strength of the magnetic layer is disadvantageous in that the magnetic layer wears in operation. The resulting powder soils the running path, and sufficient durability for still reproduction is not obtained.
In order to increase the durability of still reproduction, JP-B-28-28367 and 51-39081 (the term "JP-B" as used herein refers to a published examined Japanese patent application) propose the combined use of a fatty acid ester such as butyl stearate and a fatty acid such as myristic acid. In accordance with the working examples disclosed in the above noted patent publications, however, the coefficient of friction is increased during running under highly humid conditions such that the running tension of the magnetic tape is increased.
Although the fatty acid when used alone is effective in improving electromagnetic characteristics, if used in an amount large enough to obtain a greater lubricating effect, the magnetic layer becomes soft and is decreased in mechanical strength. The durability for still reproduction is reduced and, therefore, only a limited amount of the fatty acid can be used. In addition, the combined use of a fatty acid and fatty acid ester as described above is disadvantageous in that the running tension is increased under a high humidity condition of 85% RH (relative hymidity), although under ordinary humidity conditions such as about 50% RH the still durability is improved and the running tension is relatively low.
JP-A-52-7704 (the term "JP-A" as used herein refers to an unexamined published Japanese patent application) discloses use of a hydrocarbyl phosphoric acid ester to improve the plasticity of the magnetic recording layer, or as a dispersing agent for a magnetic paint. The addition of phosphoric acid ester to the magnetic layer increases dispersibility and somewhat improves still durability, but there is no decrease in the coefficient of friction of the magnetic layer surface.
In order to overcome the above problem, JP-A-56-80828 proposes the combined use of a saturated or unsaturated fatty acid and aliphatic hydrocabyl phosphoric acid ester in the magnetic layer. JP-A-56-80828 describes that lubricity at ordinary temperature and high humidity conditions are excellent, and that abrasive resistance and durability for still reproduction are good.
In recent years, with widespread use of commercial flexible disc drive units such as VTR, personal computers and word processors, magnetic recording media have been used under a wide variety of conditions, for example, low temperature conditions or high temperature and high humidity conditions. The magnetic recording medium is desirably not subject to changes in running durability and is stable even when used under the wide range of environmental conditions which are typically encantered. However, a satisfactory result is not obtained with the lubricant described in the above noted JP-A-56-80828.
In addition, as lubricants, higher fatty acid metal salts have been known. These lubricants, however, are not fully satisfactory with respect to reduction in the coefficient of friction, running durability, etc., under a wide range of environmental conditions. For example, JP-B-50-4121 and 52-28686 disclose that use of higher fatty acid metal salts increases lubrilicity, but use of these metal salts is disadvantageous in that crystals of the metal salt deposit on the surface of the tape.
West German Patent Application (OLS) No. 3,347,532A, European Patent Application No. 150,393A, and JP-A-60-160024 describe that the use of hydrophilic zinc stearate increases solubility and prevents the deposition of crystals on the tape surface. The hydrophilic zinc stearate, however, is not fully satisfactory with respect to decreasing the coefficient of friction and for providing good running durability, although the subject substance maintains its lubricating effect under high temperature and high humidity conditions, and under storage conditions.
Another known technique for increasing the running durability is a method in which an abrasive material (hard particles) is added to a magnetic layer. However, when that the abrasive material is added to the magnetic layer to increase the running durability of the magnetic layer, the effect of the abrasive material is not fully realized unless added in a considerably large amount. Namely, it is ultimately difficult to obtain satisfactory running durability by adding the abrasive material without a sacrifice in electromagnetic characteristics and head abrasive properties.
The present inventors have discovered that the above described problems are overcome by employing a hydroxylaluminum salt of a higher fatty acid as a lubricant.